1. Field of the Invention
The present invention is related to wireless tracking systems and methods. More specifically, the present invention relates to a system and method for mitigating multipath errors associated with the wireless tracking of objects by utilizing tags that transmit signals at various power levels.
2. Description of the Related Art
The ability to quickly determine the location of objects located within a facility is becoming a necessity of life. To the uninformed observer, the placement of transponders, also known as tags, on numerous non-stationary objects whether in an office or home would appear to be an unnecessary use of resources. However, the uninformed observer fails to appreciate the complexity of modern life and the desire for efficiency, whether at the office or home.
For example, in a typical hospital there are numerous shifts of employees utilizing the same equipment. When a new shift arrives, the ability to quickly locate medical equipment not only results in a more efficient use of resources, but also can result in averting a medical emergency. Thus, the tracking of medical equipment in a hospital is becoming a standard practice.
The tracking of objects in other facilities is rapidly becoming a means of achieving greater efficiency. A typical radio frequency identification system includes at least multiple tagged objects, each of which transmits a signal, multiple receivers for receiving the transmissions from the tagged objects, and a processing means for analyzing the transmissions to determine the locations of the tagged objects within a predetermined environment.
Several prior art references discloses various tracking systems.
Some disclose systems for determining presence, identity and duration of presence in a given area of an object.
Others disclose systems that use line-of-sight radiant wave energy for signal transmission.
Others disclose radiofrequency systems that utilize within an indoor facility and allow for an individual to be located after an alarm is triggered by the individual.
One exemplary method triangulates the strongest received signals to determine the location of a tagged object. This method is based on the assumption that the receivers with the strongest received signals are the ones located closest to the tagged object. However, such an assumption is sometimes erroneous due to common environmental obstacles. Multipath effects can result in a further located receiver having a stronger received signal from a tagged object than a more proximate receiver to the tagged object, which can result in a mistaken location determination. The prior art has disclosed various means for overcoming multipath effects.
One method discloses reducing time-shift due to multipathing for a RF signal in an RF environment.
Another method discloses indicating multipath distortion in a received signal.
Other methods disclose using feedback control interfaces that are configured to send control signals to effect corrective action for improved RFID tracking performance.
Another method discloses reducing the effects of multipath induced distortions on the accuracy of detecting the time of arrival of a received signal.
Other prior art references have disclosed the use of varying energy levels.
Kaewell, Jr. et al., U.S. Pat. No. 7,082,286 for a Path Searcher Using Reconfigurable Correlator Sets discloses producing a path profile for a user based on sorted output energy levels.
Fernabdez-Cobaton et al., U.S. Pat. No. 6,697,417 for a System And Method Of Estimating Earliest Arrival Of CDMA Forward And Reverse Link Signals discloses a mobile station receiver that detects the arrival times and energy levels of received signals, and constructs a searcher histogram and a finger histogram associated with each pilot signal.
Langford et al., U.S. Patent Publication Number 2006/0267833, for an Electromagnetic Location And Display System And Method discloses a system for near-field ranging by comparison of electric and magnetic field phase.
Chung et al., U.S. Patent Publication Number 2006/0055552, for a RFID Device For Object Monitoring, Locating, And Tracking discloses an RFID tag that transmits identifying information periodically and at different levels of transmitted power.
The prior art has yet to resolve mistaken location calculations based on multipath effects for objects tracked within an indoor facility.